1. Field of the Invention
The present invention relates to a display device, in particular, a liquid crystal display device in which a drive circuit including an inverter circuit is integrated into a panel.
2. Description of the Related Art
Thin film transistor (TFT) liquid crystal display devices in which each pixel includes a switching element have widely been used as a display device of a personal computer or the like. Also, demand for TFT liquid crystal display devices as a display device of a small-size mobile terminal such as a cell phone has been grown. TFT liquid crystal display devices are required to achieve higher image quality and reduce power consumption, as well as strongly required to reduce the cost. In particular, small-size displays for use in a cell phone are required to reduce the cost of their driver LSI (large-scale integrated circuit) for driving a panel, since the cost of the driver LSI occupies a large portion of that of each display.
In order to reduce the cost of a drive LSI, so-called “drive circuit-integrated display devices” in which high-voltage circuits such as a power supply circuit and a drive circuit, which have been integrated into a driver LSI, are formed on a glass substrate in the same process as the process of forming TFTs in each pixel have been developed and commercialized. Integration these high-voltage circuits into the panel allows a logic circuit to be formed in the driver LSI without having to undergoing a high-voltage process. Also, a shrink effect produced by process miniaturization reduces the circuit area. As a result, the cost of the driver LSI is reduced.
On the other hand, a drive circuit is formed on the panel in an N-channel metal-oxide-semiconductor (NMOS) single channel process; therefore, the process cost becomes lower than the cost of a complementary metal-oxide-semiconductor (CMOS) process. While a clock having an amplitude of the order of a dozen or so volts is generally needed in order to drive gate lines in a TFT liquid crystal display device, the amplitude of an output signal of the drive LSI is as small as the order of several volts. For this reason, a level shifter circuit is needed in order to increase the amplitude. Also, multiple clocks are needed in order to operate the integrated drive circuit; therefore, multiple level shifters are required accordingly.
As a level shifter that can be formed in an NMOS single channel process, the circuit described in U.S. Pat. No. 6,788,108 (JP-A-2003-179479) is known. However, this level shifter requires an input signal for increasing an output voltage and an inverted signal for decreasing an output voltage. Use of such a level shifter increases the number of control clock lines of the integrated drive circuit. If the integrated drive circuit also drives the common lines as well as the gate lines, the control clock line number is further increased. Generally, if a drive circuit is integrated into the panel, it is formed on a frame area outside a display area. The lines of control clocks of the integrated drive circuit are disposed in the frame area as well. Therefore, disposition of a large number of control clock lines causes a problem that the size of the frame area is increased. There also occurs another problem that the number of output pins of the driver LSI is increased and thus the cost of the driver LSI is increased.
Among conceivable methods for reducing the number of the control clock lines of the integrated drive circuit is a method of integrating an inverter circuit into the panel and generating an inverted signal to be provided to a level shifter using the integrated inverter circuit. Known as an inverter circuit that can be formed in an NMOS single channel process is the circuit described in JP-A-05-224629.
However, the above-described inverter circuit has a problem that it is significantly affected by manufacturing variations of a threshold voltage Vth, since an input circuit of the inverter circuit employs a diode connection. Specifically, the inverter circuit has a problem that a large Vth delays the rise of an output waveform and a small Vth increases current (through-current) consumption.
In a drive circuit-integrated display device, thin film transistors included in the drive circuit are formed on a glass substrate in the same process as that in which a switching element of each pixel is formed. These transistors have a problem in that they have a larger threshold voltage Vth than that of transistors used in a typical integrated circuit and a problem that there occur large manufacturing variations in Vth of these transistors. Also, these transistors have a problem that they have a larger on-resistance than that of typical transistors. Further, these transistors have a problem that if a high voltage is applied thereto or if a large current is passed therethrough, element characteristics of these transistors tend to deteriorate.